This invention relates to systems for collecting and condensing electromagnetic radiation, particularly a system incorporating parabolic reflectors for collecting radiation emitted from a radiation source and focusing the collected radiation onto a target.
The functional objective for systems that collect, condense, and couple electromagnetic radiation into a standard waveguide, such as a single fiber or fiber bundle, or outputs to a homogenizer of a projector, is to maximize the brightness (i.e., maximize the flux intensity) of the electromagnetic radiation at the target. The prior art teaches the use of so-called on-axis reflector systems involving spherical, ellipsoidal, and parabolic reflectors and off-axis reflector systems involving spherical, toroidal, and ellipsoidal reflectors. Where the target has dimensions that are similar to the size of the arc gaps of the electromagnetic radiation source, off-axis reflector systems achieve higher efficiency and brightness at the target than on-axis systems, thereby maximizing the amount of light that can be collected by a fiber optic target. For targets having dimensions that are much larger than the arc gaps of the electromagnetic source, both on-axis and off-axis reflector systems are effective for collecting, condensing, and coupling the radiation from a radiation source into a wave guide.
A disadvantage of on-axis systems is that they inherently redirect the radiation from the radiation source into larger images that are dependent on the direction of the emitted radiation, thus defeating the goal of concentrating the radiation into the smallest possible spot when the radiation source is incoherent. For example, a known on-axis ellipsoidal system produces magnifications that range from 2 to 8, dependent on the emission angle of the electromagnetic radiation. The variously magnified radiation beams are superimposed upon one another, thereby causing distortion and magnification of the collected image.
Furthermore, an ellipsoidal collecting and condensing system does not produce parallel (i.e., collimated) radiation beams. This is a disadvantage because collimated beams can facilitate filtering of the collected radiation when needed.
In known on-axis parabolic systems, the divergence of the reflected beam is also dependent on the angle of emission from the radiation source. Furthermore, such systems require the use of one or more focusing lens, which, under perfect conditions, produce a distorted image, and, in reality, typically produce badly aberrated images which effectively increase the image size and reduce brightness, or flux intensity. Furthermore, the outputs of an on-axis system are always circularly symmetric and, therefore, may not be suitable for non-circular targets.
U.S. Pat. No. 4,757,431 describes an improved condensing and collecting system employing an off-axis spherical concave reflector which enhances the maximum flux intensity illuminating a small target and the amount of collectable flux density by the small target. This system was further improved in U.S. Pat. No. 5,414,600, in which the off-axis concave reflector is an ellipsoid, and U.S. Pat. No. 5,430,634, in which the off-axis concave reflector is a toroid. Although the toroidal system described in the ""634 patent corrects for astigmatism, and the ellipsoidal system of the ""600 patent provides a more exact coupling than the spherical reflector of the ""431 patent, each of these systems requires the application of an optical coating onto a highly curved reflective surface. Applying optical coatings to such curved surfaces is expensive, and achieving a uniform coating thickness is difficult. Furthermore, in such systems the source image is focused directly from the source to the target in a relatively small space, thereby making the insertion of other optical elements, such as filters and attenuators, difficult due to the lack of space.
In the field of spectroscopy, it is necessary to focus electromagnetic radiation down to a very small spot at a sample under test and to thereafter collect the radiation reflected by the sample. Off-axis parabolic reflectors have been used for this purpose. U.S. Pat. No. 3,986,767 shows a system in which a parallel beam is focused into a small spot directly onto a sample under test using an off-axis paraboloid. U.S. Pat. No. Re 32,912 shows the use of paraboloids whereby light is focused onto a sample under test using one reflective paraboloid, and the light from that same focus is collected using a second reflective paraboloid. U.S. Pat. No. 4,473,295 describes yet another configuration for using reflective paraboloids to focus and collect radiation onto and from a sample under test.
U.S. Pat. No. 5,191,393, and its corresponding European Patent No. 0 401 351 B1, describe a system whereby light is transmitted from a location outside a cleanroom to a location inside the cleanroom for performing optical measurement of small features. One of the configurations described for collecting and transmitting light includes an arc lamp, two parabolic reflectors, a single fiber target, and transmissive dichroic filters for filtering out unnecessary wavelengths. A first parabolic reflector collects light that is reflected from the source off of a filtering reflector and creates a collimated beam. The collimated beam may pass through one or more additional filters before impinging on the second parabolic reflector, which collects and focuses the collimated beam into the single-fiber target. None of these references, however, describes a system for achieving unit magnification between the source and the focused image so as to obtain the maximum flux intensity with the minimum distortion at the target.
Therefore, there remains a need to provide a method of collecting and concentrating electromagnetic radiation using parabolic reflectors that maximizes the flux intensity of the focused radiation beam at the target.
In accordance with aspects of the present invention, an improved system for collecting and condensing electromagnetic radiation employs parabolic reflectors and achieves unit magnification, or near unit magnification, between a source image and a focused image at a target, thereby producing maximum focused intensity at the target. In particular, the present invention is directed to an optical device for collecting electromagnetic radiation from a source of electromagnetic radiation and focusing the collected radiation onto a target to be illuminated with at least a portion of the electromagnetic radiation emitted by the source. The device includes a collimating reflector and a focusing reflector. The collimating reflector comprises at least a portion of a paraboloid of revolution and has an optical axis and a focal point on the optical axis. A source located proximate the focal point of the collimating reflector produces collimated rays of radiation reflected from the collimating reflector in a direction parallel to the optical axis. The focusing reflector comprises at least a portion of a paraboloid of revolution and has an optical axis and a focal point on the optical axis. The focusing reflector is positioned and oriented with respect to the collimating reflector so that the collimated rays of radiation reflected from the collimating reflector are reflected by the focusing reflector and focused toward a target located proximate the focal point of the focusing reflector. The collimating reflector and the focusing reflector have slightly different shapes or substantially the same size and shape and may be oriented optically about symmetrically with respect to each other so that each ray of radiation reflected by a surface portion of the collimating reflector is reflected by a corresponding surface portion of the focusing reflector toward the target to achieve substantially a unit magnification.
A retro-reflector may be used in conjunction with the collimating reflector to capture radiation emitted by the source in a direction away from the collimating reflector and reflect the captured radiation back through the source (i.e., through the focal point of the collimating reflector) toward the collimating reflector to thereby increase the intensity of the collimated rays reflected therefrom.
The collimated and focusing reflectors can be arranged in an opposed, facing relationship with their respective optical axes collinearly arranged, or they can be arranged with their optical axes arranged at an angle with respect to each other, in which case a redirecting reflector is employed to redirect the collimated rays reflected by the collimating reflector toward the focusing reflector.
Alternately, the collimating reflector and the focusing reflector comprise an ellipsoid/hyperboloid pair with one of the collimating and focusing reflectors having a substantially ellipsoid shape, and the other of the collimating and focusing reflectors having a corresponding substantially hyperboloid shape with each reflector of the ellipsoid/hyperboloid pair having a corresponding size and optical orientation with respect to each other so that each ray of radiation reflected by a surface portion of the collimating reflector is reflected by a corresponding surface portion of the focusing reflector toward the target so as to preferably achieve about unit magnification between the source and an image focused onto the target. Depending on applications, larger or smaller magnifications other than one can be used resulting in reduced brightness, i.e., magnifications of from about 0.5 to about 5.
Filters or other optical elements can be arranged between the collimating and focusing reflectors.